1. Field
Various embodiments described herein relate to a driver for shaping a drive signal and an optical transmission apparatus.
2. Description of the Related Art
With recent improvement in transmission speed and the increasing capacity of networks, optical signals have been used for short range communications and medium range communications. Photoelectric conversion elements (electrical-optical converters) are employed as light sources for optical signals. The photoelectric conversion elements are compact and consume little power and that allow for direct modulation. Examples of such photoelectric conversion elements include laser diodes (LDs), such as a vertical cavity surface emitting laser (VCSEL).
In cases where an optical signal is transmitted using direct modulation of an LD, the LD is driven by a high-speed electrical signal, so that the electrical signal is converted into a high-speed optical signal. Unfortunately, there are limitations on the high-speed responsivity of the LD itself, and therefore the optical signal is sometimes not sufficiently fast even when the electrical signal is fast. Pre-emphasis is used to compensate for this shortage of high-speed responsivity of an LD. Pre-emphasis emphasizes the rising edge portion (transition from “0” to “1”) and the falling edge portion (transition from “1” to “0”) of an electrical signal for driving the LD by amplifying the rising edge portion and the falling edge portion in advance.
The specific property of an LD is that the falling edge portion is less steep than the rising edge portion because of the phenomenon of relaxation oscillations. Therefore, in the case of using pre-emphasis of symmetrically (equally) emphasizing the rising edge portion and the falling edge portion of a drive signal, the falling edge portion and the rising edge portion of an optical signal are made asymmetrical, which causes degradation in transmission characteristics. To address this issue, asymmetrical pre-emphasis of amplifying a specific portion of an electrical signal to asymmetrically emphasize its rising edge portion and falling edge portion has been proposed (e.g., see Japanese Unexamined Patent Application Publication No. 2006-40975).
In the aforementioned conventional technique, however, many transistors are used for a circuit that asymmetrically emphasizes the rising edge portion and the falling edge portion, and therefore the gate capacitance is made large. This raises a problem in that a light-emitting element cannot be driven at a high speed. For example, in the aforementioned technique disclosed in Japanese Unexamined Patent Application Publication No. 2006-40975, a circuit for processing a main signal, a circuit for emphasizing the rising edge portion, and a circuit for emphasizing the falling edge portion are used, and thus the number of transistors is large. For this reason, the gate capacitance is large, which makes it impossible to drive a light-emitting element at a high speed.